Significant challenges for a system that detects analytes (e.g., chemical and biological agents) in liquid media include concentration of the analyte in the media, and transport of the analyte to a sensor surface. For biological applications, concentration issues generally arise since the concentrations of such analytes tend to be low. Additionally, biological analytes (e.g., cells, cell fragments and macromolecules such as proteins and nucleic acids) tend to be relatively large; hence, transport issues arise because these larger analytes diffuse in fluid solution very slowly.
In addition to cells, cell fragments, and molecules such as proteins and nucleic acids, the detection of small molecule analytes can be a useful marker for diagnosing disease, monitoring drug pharmacokinetics in a patient, and for screening small molecule libraries for potential drug targets. Many therapeutic drugs, including small molecule drugs, require frequent monitoring in patients in order to maximize the beneficial effects of the drug and avoid adverse effects that may result.
For example, estradiol is a small molecule that is used to assess a number of health conditions. For example, blood tests for follicle stimulating hormone (FSH) and estradiol (E2) can be used to check ovarian reserves and for adequate egg function. In addition, estradiol and metabolites thereof can be used as a risk factor determinant for breast cancer. An elevated level of estradiol is correlated with increased risk of developing breast cancer. However, a low estradiol level is correlated with an increased risk for vertebral fractures. The U.S. Food and Drug Administration has approved the use of the drug tamoxifen in the prevention of breast cancer in certain patients. However, tamoxifen can lower serum estradiol levels to a point that increases the incidence of vertebral fracture. It is therefore desirable for a physician to be able to frequently monitor estradiol levels in a patient to ensure that estradiol levels remain within a certain range to reduce the risk of breast cancer while not increasing the risk of vertebral fracture.
Therapeutic drugs also require frequent monitoring in patients. Therapeutic drugs include, for example, immunosuppressant medications such as cyclosporin, tacrolimus (FK-506), rapamycin, and mycophenolic mofetil. Immunosuppressants have a narrow therapeutic index and high degree of inter- and intra-patient variability in bioavailability. Immunosuppressants require careful, frequent monitoring in patient samples in order to balance the level of immunosuppression needed to prevent transplant rejection while avoiding the adverse effects of excessive immunosuppression such as toxicity, infection, and increased risk of developing cancer.
The detection of small molecules such as estradiol is often difficult not only because of potentially low levels in the sample, but also because of the limited number of suitable capture agents that can be used to bind the small molecule analyte. As a result of such limitations, it is often difficult to design or modify existing assays, such as enzyme-linked immunosorbant assays that are sensitive and specific enough to detect low levels of the small molecule analyte.
Typically, detection of analytes in patient samples requires obtaining the sample in the doctor's office or clinic and sending the sample off site for analysis. Depending on the analyte, the analysis can take one day to several weeks. The results of the analysis are transmitted to the doctor, who then uses the information to adjust treatment as necessary, and contacts the patient to convey the new treatment regimen. The delay associated with analyzing a sample makes it difficult for a doctor to accurately specify a proper treatment.
There is a need for improved assays that can be used to more readily detect small molecule analytes, and to detect low concentrations of analyte. In addition, there is a need for improved measurement of analytes including small molecule analytes in order to customize drug regimens to maintain efficacy of the drug while reducing unwanted side effects in individual patients. Furthermore, there is a need for methods and apparatus that can be used at the point of care to measure biologically and/or clinically relevant analytes in order to reduce the delay between obtaining the sample and obtaining the results of the assay.
A key metric for competitive detection is the amount of analyte accumulated on a sensor per unit time. For good performance, the rate of accumulation (and the resulting signal transient) needs to be fast relative to the sensor drift rate. Another key performance metric for an analyte detection system is the degree to which the system can preferentially collect the analyte of interest on the sensor surface. Since many biological samples contain extraneous background components (e.g., other proteins, cells, nucleic acids, dirt), it is necessary to prevent these background components from interfering with the desired measurement. So, a transport method that selectively draws the analyte to the sensor and allows interfering background components to pass by has definite advantages. Such a method used in concert with selective binding of the analyte (e.g., antibody, complimentary DNA strands, etc.) to the sensor surface can deliver high sensitivity measurements for samples with large amounts of extraneous background components relative to the amount of analyte.
Various methods for improving transport of analyte to a sensor surface have been proposed, including filtration, novel flow geometries, acoustic fields, electrical fields (time varying and static) and magnetic fields.
Acoustic excitation has been used to draw cells to field nodes, but it is difficult to use this technique alone to transport material to a surface.
Electrical fields (electrophoresis and dielectrophoresis) have been used to enhance transport but are not universally applicable to all analytes and sample types. They are generally more effective for larger analytes (e.g., cells). Furthermore, the electrical properties of microbes can vary within a given species and strain, making it hard to predict system performance under all intended operating conditions. Sometimes it is necessary to tailor the ionic strength of the sample to improve the performance of the transport. This requirement can conflict with the optimum binding or wash conditions in an assay. Also, electrical fields can dissipate energy and heat conductive fluids (e.g., 0.1 M phosphate buffer solution), which is undesirable since heating can damage the biological analytes.
Immunomagnetic separation (IMS) methods are known in the art for isolating analyte from a sample.